The latest health check on wildlife is in, and it’s not pretty. A new report by two of the world’s leading environmental groups reveals that the average size of wildlife populations worldwide has shrunk dramatically, by what the report calls a “catastrophic” 73 percent in the last 50 years.
The Living Planet Report, published by the World Wildlife Fund (WWF) and the Zoological Society of London (ZSL), found that creatures living in rivers and lakes, such as the Amazon river dolphin, have experienced the most severe declines. Populations of these freshwater species have declined by an average of 85 percent, according to the Living Planet Index (LPI), a tool for measuring wildlife populations on which the report is based. Across the animal kingdom, meanwhile, wildlife populations are vanishing fastest in Latin America and the Caribbean — part of the globe that’s home to a tremendous diversity of life. The research does not include invertebrates such as insects and snails.
Losing wild animals isn’t good for anyone. Bats eat insect pests and lower the use of pesticides, yet many of them are imperiled. Parrotfish, which have declined in some regions, can improve the health of coral reefs that safeguard coastal communities. All kinds of birds and mammals help pollinate plants and spread seeds throughout the forest, sustaining forests and the rainfall they generate; many of them are vanishing, too.
“This is not just about wildlife,” Daudi Sumba, WWF International’s chief conservation officer, said on a press call Monday unveiling the report. “It’s about the essential ecosystems that sustain human life.”
World Wildlife Fund and the Zoological Society of London.
The new report is an alarming status check on the state of our planet, and the headline numbers it reveals will likely be cited in countless news and government reports. The Living Planet Index is one of the key metrics used to track global progress in efforts to conserve the environment.
Coming up with simple figures to describe the state of the world’s wildlife is inherently difficult, but more than half a dozen scientists told me that the methods used to calculate the index may cause it to overstate wildlife declines, and perhaps significantly so. Some even called it misleading. One researcher voiced concerns that if leading environmental groups exaggerate wildlife declines, it could ultimately erode trust among the public, making action on a very real crisis that much harder to achieve.
While there’s no question that biodiversity is in decline, growing criticism of WWF’s estimation casts doubt on the scale of loss — or at least on science’s ability to accurately measure it.
Scientists have poked holes in WWF’s key figure
Before digging into the report, it’s important to reiterate: There is indeed a crisis of biodiversity loss. This is unequivocal. Coral reefs are overheating and dying en masse. North America has lost some 3 billion birds. Insects are indeed vanishing. The rate of extinction is accelerating. In Hawaii, which has been called the extinction capital of the world, entire species of birds — and all the cultural heritage they carry — are blinking out as I write this.
The new WWF report underscores this frightening trend. We are living in a time of profound biodiversity loss.
Calculating a single figure to encompass all of this loss isn’t easy. Ecosystems are incredibly complex, and counting animals year after year is difficult and time-consuming. “Generating a single estimate for all species in all locations is extremely challenging,” said Laura Melissa Guzman, a quantitative biologist at the University of Southern California. “I don’t think we have reached consensus as a scientific community of what is the best way to do that.”
WWF’s approach relies on something called the Living Planet Index, which is produced by ZSL. It measures the average change in animal populations worldwide since 1970. To come up with the global LPI, scientists first calculate how individual populations of birds, mammals, reptiles, amphibians, and fish have changed, as I wrote in 2022, when WWF published its previous installment. A population of, say, 1,000 manatees that has lost 500 has decreased by 50 percent. The same is true for a population of 10 that has lost just five. Then they average up all of those changes, be they increases or decreases, to produce one number. That means the index is an average of changes in population sizes, not the average of the number of creatures lost.
This is confusing. In the past, many media stories misinterpreted figures from the index and reported that Earth lost more than two-thirds of its wildlife in the last 50 years. That’s wrong. The headline number — two-thirds, or in the case of the new report, 73 percent — refers to the average decline of thousands of different animal populations, not the total number of animals.
But a more fundamental issue than bad headlines is that the index — the basis of the new report — is not calculated correctly, according to a team of researchers at the Center for Theoretical Study in the Czech Republic, a joint institution of Charles University and the Czech Academy of Sciences.
Earlier this year, they published a paper in the journal Nature Communications that pointed out what they considered mathematical flaws in the LPI. Those flaws, the authors claim, bias the index and cause it to exaggerate the decline of wildlife.
The study takes issue with a number of things, including the math used to calculate the index and how ZSL handles poor-quality population data. That lower quality data includes population trends — ups and downs in the number of animals — based on counts from only a few points in time. For a variety of complicated reasons, the authors say, ZSL’s choices in how to calculate the index bias the data toward declines. In reality, declines and increases are more balanced, they say.
One example relates to sampling errors, cases when scientists don’t accurately report the number of animals in a wildlife population, perhaps because they’re hard to spot. If the population is small to begin with, accidentally counting fewer animals has a more dramatic, negative effect on the population trend than accidentally counting more.
“I’m really convinced that the decline of vertebrate populations is not as serious as the LPI says,” David Storch, a study coauthor and researcher at the Center for Theoretical Study, told Vox.
Several scientists who were not involved in the Nature Communications study told Vox that the concerns the article raises are valid and that biases embedded in the calculation may indeed exaggerate wildlife declines. Rodolfo Dirzo, a biodiversity researcher at Stanford University who reviewed the new report, said that while the scale of wildlife population declines is extreme, it’s likely smaller than what the WWF report suggests due to how the LPI was calculated.
“Given the broad adoption of the LPI in global biodiversity policy arena, the kind of comprehensive sensitivity analysis of the LPI that Tószögyová and colleagues performed is long overdue,” Bruce Young, chief scientist at NatureServe, a nonprofit wildlife data organization, said in an email. “I was always nervous about the certainty with which the LPI authors presented their results.”
Although an early version of the Nature Communications study was available in 2023, WWF did not tweak how the LPI is calculated in response to the paper. Storch believes there’s an incentive to keep the index as is. It’s not so much a scientific tool, he said, but a tool to raise awareness of the biodiversity crisis — and raise money.
“I see these indices more as a communication tool,” said Rahel Sollmann, a quantitative ecologist at the Leibniz Institute for Zoo and Wildlife in Berlin, who was not involved in the new report or the Nature Communications study. “I wouldn’t put too much trust into its actual numerical value.”
I posed these criticisms to WWF and scientists at ZSL. Robin Freeman, a scientist at ZSL who was involved in the LPI analysis, said he doesn’t agree that biases overestimate the measure of wildlife declines. Changing the way ZSL calculates the index to remove what some researchers see as statistical biases would have consequences of its own, he said. Freeman argues that removing less detailed or more error-prone data from the calculation — much of which is from poorer regions in the tropics where there’s less research — could fail to capture the full picture of wildlife declines.
Louse McRae, another ZSL scientist who also works on the index, added that it’s also possible that the LPI actually underestimates the scale of declines. That’s because the index tends to include more population data from birds and mammals, well-studied groups that are typically declining less than, say, reptiles and amphibians, she said.
“We do an awful lot of work to test the datasets, test the impact of outliers and extreme shifts in population,” Andrew Terry, ZSL’s director of conservation and policy, said in a Monday press briefing. Speaking about the index, he said ZSL remains “confident in its robustness.” (ZSL also published a technical document alongside the new report that details its testing.)
A spokesperson for WWF, Amy Fallah, said the main purpose of the Living Planet Report “is to raise awareness about the scale of environmental challenges facing our planet and drive urgent action that prioritizes nature in global decision-making.” That’s why the group published it ahead of COP16, a major UN biodiversity conference, where environmental leaders will meet to hash out plans for conserving nature.
“The LPR’s central aim has always been to inform and influence policy decisions that put nature at the heart of solutions to the challenges our planet faces,” Fallah said.
We know wildlife is declining. Does it matter if it’s exaggerated?
Yes and no. If the leading research and advocacy groups, such as WWF, are seen to overstate the problem, the public could lose the sense of urgency and may be less likely to lend support for what is a very real problem.
“There’s a real risk to overstating the magnitude of the biodiversity crisis,” Young of NatureServe told me. “The public could get the impression that the conservation community is prone to exaggeration and therefore calls for action to reduce wildlife loss could go unanswered.”
Young also mentioned that the “extinction denier” community — comprising people who deny that we’re living through a sixth mass extinction fueled by humans — feeds off examples, real or not, that show that wildlife is doing better than we thought. That makes it ever more important for measures of biodiversity loss to capture examples of successful conservation.
Then again, we’re just talking about shades of decline — whether the biodiversity crisis is really, really bad, or really, really, really bad. Regardless of how precise the LPI is, it portrays a trend of profound nature loss that no serious scientist disputes. “The report has been useful in bringing attention to the fact that there is a significant decline in populations of many species of vertebrates,” Dirzo said.
If the figure ultimately serves as a communication tool, it’s an effective one, judging by how widely the LPI is reported and cited. “Our goal is to use it as a tool for raising awareness about the dual crises of nature loss and climate change and urging the public and private sector to take action,” Fallah said.
And to be clear: Communication really matters. The decline of wildlife remains a somewhat fringe issue with low public awareness, even compared to other environmental concerns like climate change. This in itself is alarming, considering the sheer scale of the problem and how it impacts us all. We are talking about the ongoing collapse of ecosystems with immeasurable spiritual and cultural worth that provide human communities with clean water, healthy food, and other basic needs. They are irreplaceable.
If WWF’s new report — and the LPI index — can help communicate the problem, that ultimately seems useful.
“It shows us that we’re still not doing enough,” said Gerardo Ceballos, an ecologist at the National Autonomous University of Mexico (UNAM). “The most important thing to understand is that unless we can save biodiversity there’s no way we can save humanity.”
“People have accused me and other people of being alarmists,” Ceballos told me. “We are alarmists because we are alarmed.”
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